Neutrinos in Minnesota

Peter Litchfield

University of Minnesota

Colloquium 12th October 2005

735 km

A Short History of the Neutrino

1930 Pauli proposes the neutrino to explain decay. The first example of the particle physicist’s habit of

proposing new particles to explain any new phenomenon.

e (1956), (1962), (1975) discovered

1992 LEP says that there are only three light neutrinos with standard model interactions

1975-1998 Neutrinos are boring, distinguished mainly by what the do not have No mass, no strong interactions, no electromagnetic

interactions, no right handed interactions The standard model of particle physics is built with zero

mass, left handed weakly interacting neutrinos.

A Short History of the Neutrino

1957 Pontecorvo shows that if neutrinos have mass and more than one species (flavour) exists they may oscillate from one flavour to another as they travel through space. However there is no evidence that this happens.

1968 The first fly in the ointment is Ray Davis’s observation of a deficit of neutrinos from the sun but nobody believes this is due to neutrinos having mass and oscillations.

~1980 Grand Unified Theories predict that proton decay may exist at measurable rates. An industry of large underground detectors is born.

1988 Proton decay is not discovered but the IMB experiment notes that they find fewer of their background interactions than predicted. This is followed by Kamiokande and Soudan 2, but nobody believes that it is due to neutrino oscillations.

A Short History of the Neutrino

1998 Super-K at last has unequivocal evidence for atmospheric neutrino oscillations by detecting the a difference between the rate of upward and downward going neutrinos, later confirmed by Soudan 2.

2002 SNO detects the predicted rate of neutral current interactions of solar neutrinos, the solar standard model is correct and neutrinos are oscillating

Today Everybody believes that neutrinos have mass and oscillate. The first physics “beyond the standard model”

Neutrino Phenomenology

We assume that there are three neutrinos (if there are four and LSND is right, things are more complicated yet)

Neutrinos can be described as eigenstates of flavour (e,,) or of mass, they are not necessarily the same.

The flavour eigenstates (e,,) are a mixture of the mass eigenstates (1,2,3)

When they are produced neutrinos are eigenstates of flavour, e.g.

When neutrinos propagate they do so as the mass eigenstates This is what produces neutrino oscillations

i

iiU

Why do Oscillate?

Quantum mechanical phenomenon, not a special property of neutrinos

Initial state has pure flavor, e.g.

But is a mixture of mass states

Each mass state has the same initial energy but different mass, therefore different velocity

After traveling some distance the particle wave packets will have changed phase Now a different mixture of mass states

Therefore a different mixture of flavor states

1

2

3

+e+

Time t Distance L later

Three Neutrino Phenomenology

The Matrix U can be decomposed into three submatrices with elements which are the sines and cosines of 3 angles 12, 13, 23 and a phase (responsible for CP violation)

MINOS NOA

Two neutrino oscillations

At short distances (~100s of kilometers) the atmospheric data says that to a good approximation oscillate to as though there were just two mass states m2 and m3

Quantum mechanics says after oscillation the probability of a remaining a is

Where L is the distance traveled E is the neutrino energy m23

2 is the mass squared difference (m32-m2

2)

sin2223 defines the amplitude of the oscillations

The full formalism for three neutrinos is more complicated (see later)

E

LmP

2232

232 27.1

sin2sin1

What we know today

Solar e oscillate to ( SNO, Super-K, Kamland, GNO)Atmospheric oscillate to not to e

(Super-K, K2K, CHOOZ)3 is an approximately equal mixture of and with only a small, as yet unmeasured, amount of e

the compositions of 1 and 2 are well determined from the solar dataWe know nothing of the sign of m2 or of

Why do we want to know?

The standard model is very successful BUT Why this set of fundamental particles? Why do they have these masses? Why are they mixed together in the way they are?

The best guess is that at very high energies they are governed by a fundamental symmetry which is broken at low energies. The patterns of the breaking may give clues to the underlying symmetry

CP violation in the weak interaction may be the origin of the matter-antimatter asymmetry of the universe.

Neutrinos may give the clue leading to the theory of everything!!

b s d

tcu

τμe ν ν ν

μ τ eQuarks Leptons

THE fundamental

particles

The Experiments

Verification of the oscillation model (MINOS)

Better determination of parameters (MINOS)

Detection of →e (13) (MINOS, NOA)

Determination of sign of m2

(NOA)

Observation of CP violation () (NOA?)

MINOS

NOA

Receipe for a Neutrino Experiment

First make your neutrino Neutrinos are produced in the

weak decay of particles, , , K, n Neutrinos do not interact very

often, therefore we need to make a lot of particles, particularly

The Fermilab New Main Injector (NuMI). 120 GeV, high intensity injector for the collider complex. Currently the most powerful accelerator in the world in terms of the energy delivered to a target and thus the number of secondary particles produced.

The Fermilab Neutrino Beam

Need to make a beam directed at your detector Neutrinos are neutral and don’t interact, they cannot be focused Produce secondary particles (,K mesons) by the proton beam hitting a target Focus the secondary particles into a beam, then when they decay the neutrinos

will follow approximately the beam path. Allow the secondary particles to decay The decay length of a 10 GeV is 560m. Need a long decay volume. Finally need an absorber to get rid of everything in the beam except neutrinos.

Protons | p + + K + + | + |

1000m

Beam Components

The Fermilab Neutrino Beam

Focusing horns Attempt to produce a parallel beam of secondary particles Produce a very high magnetic field between inner and outer

conductors Arranged such that particles produced at a large angle see the most

field and are thus bent most towards the beam axis Very high currents ~200kA, therefore pulsed Selects one sign of secondary and thus produces a mostly neutrino or

anti-neutrino beam

Recipe for a Neutrino Experiment

Next catch your neutrino Neutrinos don’t interact very often

Number of interactions is proportional to the number of nucleii in your detector.

Need a very massive detector to give enough interactions.

Want to detect and measure the directions and energies of the outgoing particles in a neutrino interaction. Need a magnetic field to measure the outgoing particle momenta by

curvature. Need fine segmentation to give accurate determination of particle

trajectories.

Fine segmentation and a lot of mass are very expensive Need to have a detection system that collects information from a

large volume cheaply.

Recipe for a Neutrino Experiment

Observe Oscillations Measure the composition of the beam in a detector at Fermilab where

the beam is produced (Near detector). Beam is intense and narrow, the detector can be relatively small but must

be able to distinguish interactions produced in the same beam burst.

Allow the beam to propagate to Minnesota where the composition is measured again (Far detector). Beam is broad (~km) and weak, the detector must be as large as we can

afford to give sufficient events

If the composition is different the neutrinos have oscillated Observe oscillation structure in the energy distribution

The MINOS Collaboration

Minos collaboration members at Fermilab with the Near Detector surface building in the background (right)

175 physicists from 31 institutes in 5 countries

Argonne – Athens – Brookhaven – Caltech – Cambridge – Campinas – Fermilab – College de France – Harvard – IIT – Indiana – ITEP Moscow – Lebedev – Livermore – Minnesota, Twin Cities – Minnesota, Duluth – Oxford – Pittsburgh – Protvino – Rutherford Appleton – Sao Paulo – South Carolina – Stanford – Sussex – Texas A&M – Texas-Austin – Tufts – UCL – Western Washington – William & Mary - Wisconsin

U.K.

U.S.A.Greece

Russia

BrazilFrance

MINOS Timeline

The Soudan 2 collaboration had the first thoughts of a long baseline neutrino experiment at Soudan around 1989. The detector would be Soudan 2. But the Main Injector was still years in the future

However it became obvious that Soudan 2 was too small and the MINOS collaboration formed in 1994 to design and construct a new bigger detector

Final approval was given in 1998 and construction started

The far detector was completed in 2002 and started collecting data on atmospheric neutrinos and cosmic ray muons

The near detector and the beam were completed at the end of 2004

First data March 2005

The experiment is now running smoothly, first results next year.

You need your health and strength and lots of patience

MINOS Technology

The MINOS active element is a solid scintillator strip 4.1x1x800 cm3

Emits a flash of light when traversed by a particle

Photons are absorbed in a fiber glued to the strip

Photons are re-emitted at a different wavelength and propagated along the fiber by total internal reflection

Photons are detected by a multi-anode photomultiplier

8 fibers go to each of 16 pixels, each photomultiplier reads out 40m3 of detector

Detector

Technology

Special Thanks M. Proga

2.54cm Steel absorber(2.50cm in CalDet)

WLS Fibers

Multi-anode PMT

Fiber ''cookie''

Scint. Plane

Readout Cable

PMT DarkBox

MINOS Far Detector

192 strips, making a 8mx8m hexagon of active detector are sandwiched between 2.5cm steel plates

The steel acts as a target for the

neutrinos Has a toroidal magnetic field

produced by a coil passing through the center to measure outgoing muon momenta

Total mass of the detector 5400 tons

MINOS Near Detector

The Near detector is as nearly identical as possible so that detection inefficiencies cancel between the two. But the beam is much smaller the rates are much higher

1000 tons mass

Can do lots of conventional neutrino physics as well as oscillation physics

A much finer grained detector (MINERA) is planned to go in front.

What do we expect to see?

A interacting produces Either a meson, a non-interacting

track, plus hadrons, which make a shower of hits in MINOS (charged current interaction)

Or a , which we don’t see, plus a hadron shower (neutral current interaction)

A e interacting produces Either an electron, which produces a

denser shower of hits, plus hadrons Or a e plus a hadron shower

There can be 0→ in the hadron shower in neutral current events which produce electron showers and can be misinterpreted as e cc events

Hadron shower

Simulated events

e

e

We do have real evemts

The very first Fermilab neutrino seen at Soudan A interacted in the rock upstream of the detector and sent a

into the detector

Events in the near detector

Lots of neutrinos interact every beam pulse in the near detector The software programs have to sort them out

Already > 100,000 events obtained

They are used to understand the beam composition and properties and the analysis programs

What do we expect to find?

We are doing a “blind analysis” so we cannot show yet far detector data. I show simulated data that we expect at the end of the experiment

We will measure the ratio of the energy spectra measured at Fermilab to that measured at Soudan

If the neutrinos have oscillated there will be a deficit at Soudan which peaks at the oscillation maximum (m2)

The depth of the deficit gives sin2223

What do we expect to find?

We will search for →e, signaled by an excess of events with an electron in the final state, compared with the small (~0.5%) background of e in the beam.

MINOS is not designed for electron detection so the sensitivity only improves by approximately a factor of 2 on the current limit from reactor experiments , we need the follow-up NOA experiment….

m2=0.0025eV2

sin2213=0.067

25 x 1020 protons on target

3 limits for various exposures

What Next?

MINOS will confirm (or otherwise) that the phenomenon observed in atmospheric neutrinos is neutrino oscillations and improve the parameter measurements.

Next we want to investigate the rest of the neutrino parameters by studying in detail →e NOA

Competition: T2K experiment in the Super-K detector in Japan

15.7 m,384 cells

15.7 m,384 cells

132 m, 1984 planes

8 planes

8 planes,each with 8 cells

Magnification of ~ 30 xMagnification of ~ 30 x

15.7 m,384 cells

15.7 m,384 cells

8 planes

8 planes,each with 8 cells

Magnification of ~ 30 xMagnification of ~ 30 x

132m 1984 planes

Formalism →e Oscillations

Probability of oscillating to e in vacuum:

P=P1+P2+P3+P4

P1=sin2θ23sin22θ13sin2(1.27m132L/E) “atmospheric”

P2=cos2θ23sin22θ12sin2(1.27m122L/E) “solar”

P3= Jsinδsin(1.27m122L/E)

P4=Jcosδcos(1.27m122L/E)

J=cosθ13sin2θ12sin2θ13sin2θ23sin(1.27m122L/E)sin(1.27m13

2L/E)

“atmospheric- solar interference”

The P1 term involves the atmospheric oscillation length (m132) and

is the dominant term

One probability, two unknowns, therefore ambiguities, need extra information to solve for all the parameters.

Matter effects

Matter is not CP invariant, it is made up of matter, not antimatter. Neutrinos passing through matter interact differently than anti-neutrinos

All flavours of neutrinos can interact with electrons via Z exchange, only e interact via W exchange

In matter at oscillation maximum, P1 will be approximately multiplied by (1 ± 2E/ER) where the + sign is for neutrinos with normal mass hierarchy and antineutrinos with inverted mass hierarchy. ER11GeV for the earths crust

About a ±30% effect for NuMI, but only a ±11% effect for T2K .

If sin2213 is big enough we can determine the sign of m2

e

e

e

eW

x xZ

e e

How to study →e

NOA needs to detect final state electrons from interactions of oscillated e

and separate them from neutral current events producing a 0→ Needs to be less dense and more active then MINOS, so

event details are revealed Needs to have long radiation length so that conversions

are separated from the event vertex.

Get rid of the iron in MINOS and make a detector out of liquid scintillator in extruded plastic cells

Read out the cells with an APD and a looped fiber Cheaper and more sensitive than MINOS

XeNe

15.7 m

3.9 cm 6 cm

A Gigantic Experiment

It also has to be big since we know that sin2213 is small and thus the event rate low Needs to be 30,000 tons, 6 times bigger than MINOS and much less dense 15m x 15m x 132m 761,856 of 3.9 x 6 x 1500cm cells, 80% active Held together by plastic and epoxy

BaBar CDF DZero CMS ATLAS at about the same scale

15.7 m

15.7 m

NOA

132 m

15.7 m

15.7 m

NOA

132 m

15.7 m

NOA

132 m

The Off-axis Beam

MINOS will tell us an approximate value of m2 in the next few months.

The NuMI beam was designed to have a broad energy spread since we did not know m2 and thus the oscillation energy, and we want to map out the oscillation spectrum as a function of energy.

Now we would like to tune the beam to the oscillation energy (~2GeV at 750-800km distances). Maximizes the appearance event rate Minimizes the background from high energy neutral current events

We want to be as far from Fermilab as possible to maximize matter effects and sensitivity to the sign of m2.

The NuMI beam points directly at Soudan

WE CAN USE THE SAME BEAM FOR NOA IF WE PLACE IT OFF THE MAIN AXIS OF THE BEAM!

The Off-axis Beam

Its all down to kinematics

In the pion rest frame

and energies determined by energy conservation

In the lab frame

energy depends on the boost and the angle between the and

The Off-Axis beam

At 10 km off axis at ~800km compared to the zero degree beam we have; ~5 times the event rate around

the oscillation maximum Very much smaller high

energy component, very much reduced neutral current background

A proton driver (new high intensity injector for the Main Injector) at Fermilab could increase the rates by at least a factor of 5

Where should we put it?

It should be as far from Fermilab as possible

The Ash River site in Northern Minnesota is 810 km from Fermilab and 12km from the center of the beam

Also there will be a much smaller near detector on the Fermilab site

Soudan

Orr-Buyck

Typical events

e-

p

+

e

p

-

+

p

o

What do we expect to find?

At the current limits on sin2213 we should obtain ~100 electron events from a three year beam run

The limits on sin2213 are dependent on sign(m2) and We will be sensitive to sin2213 if it is greater than 0.01-0.02 with the current beam and 0.003-0.01 with a proton driver

Limits are comparable or better than those of the competition, the Japanese experiment, T2K

3 sensitivity to sin2213

2.5 years each of and

Sign of m2

To measure sign m2 we have to run with both neutrinos and anti-neutrinos.

Matter effects on passing through the earth are of opposite sign depending on the sign of m2

Measure the asymmetry between the two cases.

Again the sensitivity depends on T2K has a much shorter baseline (295km) and thus is not very sensitive to this sign

95% confidence limit on sign(m2)

CP violation ()

CP violation is the weakest of the effects in neutrino oscillations

The matter effect produces an apparent CP violation

Need at least two measurements to resolve the two effects

NOA + T2K, different matter effects

Only sensitive to if we are very lucky with the parameters AND both experiments have proton drivers

Opportunity for a SUPERNOA experiment, 100ktons of liquid argon at the second oscillation maximum?

NOA Timeline

Now, recommended approval by the PAC and great support from the Fermilab management

Now, Detector R&D ongoing

Autumn 2006, project approval

Summer 2009, start detector construction

Spring 2010, 5ktons (MINOS size) operational

Summer 2011, detector completed

3 Sensitivity to sin2(213)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017

Start of Fiscal Year

sin

2 (21

3)

NOA assumingproject start inFY2007

T2K assuming a 50 GeV synchrotron and completionof the 400 MeV Linac inJFY2010

m322 = +0.0025 eV2

sin2(223) = 1.0typical

3 sensitivity to sin2213

Summary

A long term program of neutrino physics is under way in Minnesota Soudan 2

Confirmation of atmospheric neutrino anomaly, probably oscillations

MINOS Measurement of oscillation parameters in → oscillations

Rule out alternative explanations of atmospheric neutrino effect

NOA Observe →e oscillations

Measure sin2213, sign(m2), CP violation parameter

First physics beyond the standard model, hopefully a window on the ultimate theory of everything

Minnesota is at the cutting edge of particle physics today